To add a image description is pretty easy. All you have to do is to add a multi line text column named LongDesc (if not done yet) to the image library you want to have a description on. Then add your description.

The BKSImage renders your image with a LONGDESC attribute that contains a reference to a page which renders the content of the image's LongDesc Field. If you leave the LongDesc column of the image ListItem blank, no LONGDESC attribute is rendered.